1. Field of the Invention
The present invention relates to voltage controlled oscillators and in particular, to inductive-capacitive (LC) based quadrature voltage controlled oscillators (VCOs).
2. Description of the Related Art
Radio frequency (RF) transceivers which rely on image rejection require accurate quadrature signals. One well known technique for generating quadrature signals involves the use of two symmetrical LC-tank VCOs (LC VCOs) so as to obtain the benefit of good phase noise performance of LC oscillators. Such a VCO implementation is often referred to as a quadrature VCO (QVCO).
Referring to FIG. 1, such a QVCO 10 is generally implemented as a two stage design with each stage A1, A2 providing 90 degrees of phase shift. The two stages A1, A2 are substantially identical and are connected in such a way that they effectively prevent each other from oscillating when their relative phase is not in quadrature. As a result, these coupled oscillators A1, A2 synchronize to the same frequency regardless of mismatches in the resonant circuitry inside (discussed in more detail below).
As indicated, the noninverting and inverting output terminals of the differential output of the first stage A1 are connected directly to the differential input terminals of the second stage A2, i.e., to the corresponding noninverting and inverting input terminals. Conversely, the noninverting and inverting output terminals of the differential output of the second stage A2 are cross coupled to the differential input terminals of the first stage A1, i.e., the noninverting and inverting output terminals of the second stage A2 are connected to the inverting and noninverting input terminals of the first stage, respectively.
As a result of these direct and cross coupled connections, a differential, i.e., out of phase by 180 degrees, phase relationship is maintained between the noninverting and inverting terminals at the inputs and outputs of both stages A1, A2, and a 90 degree phase shift, i.e., a quarter wavelength of the oscillation frequency, is maintained between the corresponding input and output terminals. Hence, the differential output signals Vi (Vi+, Vi−), Vq (Vq+, Vq−) are in quadrature, i.e., are 90 degrees, or a quarter wavelength, apart with the I-phase signal Vi either leading or lagging the Q-phase signal Vq.
Referring to FIG. 1A, one example embodiment 20 of a VCO circuit for the stages A1, A2 of the circuit 10 of FIG. 1 includes a differential amplifier circuit and a resonant circuit in the form of a dual LC tank circuit. For example, N-type metal oxide semiconductor field effect transistors (N-MOSFETs) N1a and N1b are connected in a differential manner, and are biased with a tail current provided by N-MOSFET N2 which, in turn, is biased by a bias voltage Vbias. Additional transistors N3a, N3b are connected across the differential transistors N1a, N1b and are cross coupled via their respective drain and gate terminals, as shown. Symmetrical LC tank circuits are connected such that the power supply voltage VDD is applied to the differential amplifier transistors N1a, N1b, N3a, N3b through the inductors L1, L2. The tank capacitors C1, C2 are serially connected in a shunt arrangement across the output terminals OUT+, OUT−.
The inductors L1, L2 resonate with the capacitors C1, C2, as well as with junction capacitances between the gate and drain terminals of transistors N3a and N3b. This determines the frequency of oscillation. (It will be understood that the capacitors C1, C2 can be implemented as varactors so as to allow control over the frequency of oscillation.) The negative resistances of transistors N3a and N3b counteract losses within the inductors L1, L2. (Alternative forms of LC based VCO circuits, many of which are well known in the art, can be used as well, since the specific structure of such circuit is not material to the presently claimed invention.)
A problem with this conventional QVCO circuit 10, however, is that the two stages A1, A2 can provide either a positive or negative 90 degree phase shift, thereby potentially producing two different frequencies of operation as well as an indeterminate, i.e., probabilistic, phase relationship between the I-phase and Q-phase signals. Circuit applications requiring a known, or determinate, phase relationship in the quadrature output signals cannot tolerate this.
This chance for either a positive or negative quarter wavelength phase shift can be explained as follows. The total phase shift through the loop will be zero degrees, or a multiple of 360 degrees. If x is the phase shift provided by one VCO stage A1/A2, then this phase shift can be described as follows:x+x+180=0 or 360  (1)x+x+180=360=>x=+90  (2)x+x+180=0=>x=−90  (3)
Accordingly, the I-phase signal can lead or lag the Q-phase signal by 90 degrees. This indeterminate phase relationship between the I-phase and Q-phase signals presents a problem for a number of applications, including half-rate clock and data recovery systems which require quadrature clock signals. Such systems require the I-phase signal to lead the Q-phase signal to ensure correct data recovery. Similarly, RF receivers will produce either the lower sideband or upper sideband of an up-converted or down-converted signal depending upon the phase relationship of the I-phase and Q-phase signal. An incorrect phase relationship will produce the wrong sideband.